1. Field of the Invention
This invention relates to a shared unit selection system for an information processing system, and more particularly to a shared unit selection system for an information processing system, which ensures a correct operation of a shared unit in accordance with priority of control units through the use of extremely simple circuit construction and enables an increase in the number of control units without increasing the cost of the selection system.
2. Description of the Prior Art
With the necessity of higher speeds and enhancement of reliability of an information processing system, the construction of the system has become more and more complicated. For example, a magnetic tape unit and magnetic tape control units may be employed as a shared unit and sharing units respectively. If they are connected so that one magnetic tape unit is shared by more than two magnetic tape control units, in such a manner as to be accessible from any one of the magnetic tape control units, the following advantages can be obtained.
1. The efficiency of utilization of one magnetic tape unit can be improved and information processing can be increased in speed. PA1 2. If one of the magnetic tape control units gets out of order, the magnetic tape unit can be accessed from the other magnetic tape control units, thus diminishing the possibility of the system becoming inoperative, thereby providing enhanced reliability. PA1 3. In order to provide a standby unit, it is sufficient to provide only one standby unit for more than two magnetic tape control units, so that the system can be constructed at low cost.
The advantages are great, but in the case where the magnetic tape unit is connected to be accessible from any out of a plurality of magnetic tape control units and the magnetic tape unit is accessed by two or more of the magnetic tape control units at the same time, due to time lag of clocks and response propagation of the magnetic control units, different instructions may be simultaneously given to the same magnetic tape unit thereby causing an erroneous operation. Accordingly, it is necessary to assign priority to the magnetic tape control units.
FIG. 1 is a diagram showing the system construction of a conventional selection system. In FIG. 1, reference character CTL indicates control units (for example, magnetic tape control units), used as sharing units; and DV designates units to be controlled (for example, magnetic tape units), used as shared units.
In accessing the shared units DV, a scanning system has heretofore been employed. The scanning element is connected in common with the plurality of control units CTLA and CTLB. In the system where the shared units are accessed by scanning, when the scanning element is out of order, all of the control units are also inoperable, thereby increasing the probability of down-time.
Further, it is also a very difficult problem of deciding how a power source is to be connected to the scanning element. If the power is supplied from a specified one of the control units CTL, the above disadvantage is encountered when the scanning element gets out of order. Alternatively, power can be supplied from a plurality of units, in common, but this method has the disadvantage of being complex in circuit construction.
Moreover, in the conventional case, where it is desired to increase the number of control units CTL, the new units cannot be easily connected to the scanning part, without modification of the circuit construction, and this modification leads to an increase in the cost of the system.